Linac Flashcards
How does the flattening filter affect the photon beam?
The flattening filter is used in photon beams to create a flat dose profile at a depth of 10 cm in water. Without it, the beam profile is very peaked. The flattening filter produces horns at the beam edges at shallower depths. It is typically a conical piece of high-Z material. Due to its introduction to the beam,
it increases the effective energy of the beam, by filtering out low energy photons (beam hardening) but it decreases the beam intensity and thus the dose rate.
What changes are made to switch from photon mode to electron mode in a linear accelerator (linac)?
In electron mode, the target and flattening filter are removed. A scattering foil is added to create a broad, flat beam. An electron applicator is used to collimate the beam by removing the scatter in air. The accelerated beam current is much lower in electron mode, as there is no loss in efficiency as seen in photon mode due to the presence of the target and flattening filter.
How does an electron gun work?
An electron gun is a simple electrostatic accelerator containing a heated cathode filament and a perforated grounded anode. A hot cathode emits electrons, which are accelerated toward an anode, passing through an aperture to reach the accelerating waveguide. Along this path, negatively charged focusing electrodes narrow the electrons into a fine beam which then passes through the aperture in the anode. A grid allows for synchronization of electron release into the accelerating waveguide to match its phase.
Why is a bending magnet used in a linear accelerator (linac)?
The bending magnet redirects accelerated electrons toward the isocenter. Using just a 90° bending magnet in the design of a linac allows the accelerator to be oriented parallel to the floor so it can more easily rotate around its isocenter without hitting the ceiling, floor or patient. Using a 270° bending magnet, also allows the energy of electrons to be selected and the electron beam to be focused, thus improving the penumbra of the beam.
What is the advantage of a standing wave over a traveling wave accelerating waveguide?
Standing wave accelerating waveguides are more efficient and shorter than equivalent traveling waveguides. To create a standing wave, the microwaves are reflected at the end, rather than exiting the waveguide. This generates a superposition of the waves in every second cavity, providing double the accelerating power of a traveling wave cavity. The interleaving cavities, where the reflected microwaves cancel are shifted to the side of the electron path as they do not provide any acceleration. Thus, the accelerating power is increased, and the waveguide is shortened.
What is a klystron?
A klystron amplifies low power microwaves into high power microwaves. As electrons are sent though a drift tube, their velocity is modulated by the alternating electric field at the frequency of the entrant low power microwaves, creating “bunches” of electrons. The bunches induce charges on the end cavity, creating higher power microwaves at the same frequency.
What is a magnetron?
A magnetron is a microwave generator. It has a circular structure with a cathode at the center and an anode at the outer surface made up of resonant cavities. Electrons are produced at the cathode and are subjected to an electric field between the anode and cathode. A static magnetic field is applied perpendicular to the electric field and motion of the electrons. The electrons move in spirals toward the cavities, creating microwave power, which is then sent to the accelerating waveguide.
What are some advantages of using a flattening filter free treatment beam?
The dose rate can be three to five times higher, which results in faster treatments. The absence of a flattening filter significantly reduces the amount of scattered radiation. This can reduce the total body dose to the patient and also make it easier to model the beam in a treatment planning system.
What are the disadvantages of using a flattening filter free treatment beam?
The beam profile is forward peaked. The beam is only meant to treat small targets. Very large targets would require more monitor units to treat the tumor volume that is far away from the central axis. The beam is also slightly less penetrating than a flattened beam which is hardened by the presence of the flattening filter.
How does the inverse square law relate to absorbed dose from a linear accelerator (linac)?
The absorbed dose factor is inversely proportional to the square of the distance from the target. This is due to the divergence of the beam. Imagine a sphere around the target with radius r. The surface of the sphere is 4π r2, with all the radiation from the target passing through it. Now imagine a sphere twice as far away from the target, the same amount of radiation will pass through its surface, but the surface area is four times the size, effectively reducing the intensity at any one point by four.
What is the advantage of proton and heavy charged particle radiation compared to photon and electron beams?
The major advantage of high-energy protons and other heavy charged particles is their characteristic dose distribution as a function of depth. As the beam traverses the tissues, the dose deposited is approximately constant with depth until near the end of the range where the dose peaks followed by a rapid falloff to zero. The region of high dose at the end of the particle range is called the Bragg peak.
